1. Field of the Invention
The invention relates to medullary reaming systems and to flexible drive shafts therefor.
2. Description of the Prior Art
Medullary reamers are used to enlarge the medullary canals of bone for various reasons. The medullary canals of bone typically have some degree of curvature and, for this reason, are almost always prepared with reamers having a flexible shaft.
One type of prior flexible medullary reamer shaft consists of a spiral or helically wound metal wire or strip which comprises the shaft of the reamer. A disadvantage of this type of shaft is that the reamer can be operated only in the forward mode of rotation. If the reamer is reversed, which is occasionally necessary in order to free a lodged reamer, for example, the shaft unwinds, damaging the shaft. Another disadvantage of this spiral shaft design is that the voids between the shaft coils can trap blood and tissue, making it extremely difficult to thoroughly and properly clean and sterilize the shaft after use. Another disadvantage is that if the cutting head experiences unusually high resistance, the driving torque will accumulate in the shaft as its coils close and then, when it overcomes the resistance to the head, will be released in a sudden burst, causing the cutting head to jump or spin ahead rapidly in an uncontrolled fashion. Such irregular movement of the cutting head may damage the bone.
Another type of medullary reamer shaft comprises a plurality of parallel, flexible elements joined together at their opposite ends by means of a welded or soldered connection. Such a shaft construction suffers from most of the same disadvantages as the helically coiled shaft described above. Another disadvantage occurs in attempting to utilize the central bore of the reamer, to receive a long, small diameter guide wire, which had previously been inserted into the medullary canal to act as a track for the advancing reamer. Except at its respective ends, this parallel-element reamer shaft lacks a well-defined and bordered central bore, making it difficult to prevent the guide wire from exiting the reamer in the area of the free standing shaft wires during the initial positioning of the guide wire within the reamer.
To overcome many of these disadvantages, there has also been provided a hollow tubular shaft formed of synthetic plastic material or a fiber-reinforced composite material. However, plastic shafts may lack the necessary torsional strength. Also, the reamer is autoclaved often and plastic will ultimately fail. A disadvantage of fiber-reinforced composite shafts is that, on failure, there is a danger that fibers will enter the blood stream.
Also, in prior medullary reamers the cutting head has been fixed to the flexible shaft, permanently by suitable bonding or the like. Thus, the head and the shaft form an integral unit and, when it is desired to use a different size cutting head, an entire reaming unit must be substituted. It is known to attach cutting heads to the shaft by suitable fasteners, such as a set screw, but this requires handling and the use of suitable tools.